Switching regulators have been widely applied in various power supplies and drivers, to convert an input voltage to a regulated output voltages by switching one or more power switches. For example, FIG. 1 shows a driver for providing a positive voltage VOUTP and a negative voltage VOUTN for driving an active matrix organic light emitting diode (AMOLED) panel, in which a transistor LV_P1 is connected between a positive voltage output terminal VOUTP and a switching node LXP, a transistor LV_N1 is connected between the switching node LXP and a voltage input terminal PGND, a controller 10 switches the transistors LV_P1 and LV_N1 to charge and discharge an inductor L1 connected between the switching node LXP and a voltage input terminal Vin to generate the positive output voltage VOUTP, a transistor HV_P2 is connected between a voltage input terminal VINP and a switching node LXN, a transistor HV_N2 is connected between the switching node LXN and a negative voltage output terminal VOUTN, and a controller 12 switches the transistors HV_P2 and HV_N2 to charge and discharge an inductor L2 connected between the switching node LXN and a ground terminal GND to generate the negative output voltage VOUTN, In this driver, the voltage conversion circuit 14 to generate the negative output voltage VOUTN operates to convert a positive input voltage VINP, e.g. 3.3V, to the negative output voltage VOUTN, e.g. −5.5V, and thus the transistors HV_P2 and HV_N2 will encounter high voltage thereacross during their operation, which requires use of high voltage transistors for the transistors HV_P2 and HV_N2. However, a high voltage transistor has larger parasitic capacitances between its gate and source and between its gate and drain, and thus the high voltage transistors HV_P2 and HV_N2 will bring significant switching loss and thereby result in worse efficiency of the driver, and more worse in light loading of the driver. FIG. 2 shows an efficiency curve of the driver shown in FIG. 1 for converting a positive input voltage VINP of 3.7V to a negative output voltage VOUTN of −5.5V, wherein the Y axis represents the percentage of the efficiency, and the X axis represents the load current which reflects the loading variation. As shown in FIG. 2, the maximum efficiency of the driver shown in FIG. 1 for the negative voltage conversion is only 80%.
Therefore, it is desired a circuit and a method for improving the efficiency of such type of drivers.